Method for replicating optical elements, particularly on a wafer level, and replicas formed thereby

ABSTRACT

Integrated multiple optical elements may be formed by bonding substrates containing such optical elements together or by providing optical elements on either side of the wafer substrate. The wafer is subsequently diced to obtain the individual units themselves. The optical elements may be formed lithographically, directly, or using a lithographically generated master to emboss the elements. Alignment features facilitate the efficient production of such integrated multiple optical elements, as well as post creation processing thereof on the wafer level.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 09/503,249 filed Feb. 14, 2000, now U.S. Pat. No. 6,610,166 issuedAug. 26, 2003, which is a continuation application of U.S. applicationSer. No. 08/943,274 filed Oct. 3, 1997, now U.S. Pat. No. 6,096,155 toAug. 1, 2000, which are hereby incorporated by reference in theirentirety for all purposes.

This application claims the benefit of priority under 35 U.S.C. §120 toU.S. application Ser. No. 08/727,837, filed Sep. 27, 1996, entitled“Integrated Optical Head for Disk Drives and Method of Forming the Same”and U.S. application Ser. No. 08/917,865, entitled “Integrated BeamShaper and Use Thereof” filed Aug. 27, 1997, which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention is directed to integrating multiple opticalelements on a wafer level. In particular, the present invention isdirected to efficient creation of integrated multiple elements.

BACKGROUND OF THE INVENTION

As the demand for smaller optical components to be used in a widervariety of applications increases, the ability to efficiently producesuch optical elements also increases. In forming such integratedmultiple optical elements at a mass production level, the need foraccurate alignment increases. Further, such alignment is critical whenintegrating more than one optical element.

Integrated multiple optical elements are multiple optical elementsstacked together along the z-axis, i.e., the direction of the lightpropagation. Thus, light travelling along the z-axis passes through themultiple elements sequentially. These elements are integrated such thatfurther alignment of the elements with themselves is not needed, leavingonly the integrated element to be aligned with a desired system,typically containing active elements.

Many optical systems require multiple optical elements. Such requiredmultiple optical elements include multiple refractive elements, multiplediffractive elements and refractive/diffractive hybrid elements. Many ofthese multiple element systems were formed in the past by bondingindividual elements together or bonding them individually to analignment structure.

In bulk or macroscopic optics to be mounted in a machined alignmentstructure formed using a mechanical machining tools, the typicalalignment precision that can be achieved is approximately 25-50 microns.To achieve a greater level of 15-25 microns, active alignment isrequired. Active alignment typically involves turning on a light source,e.g., a laser, and sequentially placing each optic down with uncuredultra-violet (UV) adhesive. Then each part is moved, usually with atranslation stage, until the appropriate response from the laser isachieved. Then the part is held in place and the epoxy is cured with UVlight, thereby mounting the element. This is done sequentially for eachelement in the system.

Alignment accuracies of less than 15 microns for individual elements canbe achieved using active alignment, but such accuracies greatly increasethe amount of time spent moving the element. This increase is furthercompounded when more than one optical element is to be aligned. Thus,such alignment accuracy is often impractical even using activealignment.

In many newer applications of optics, as in the optical headconfiguration set forth in commonly assigned co-pending application Ser.No. 08/727,837, which is hereby incorporated by reference, and theintegrated beam shaper application noted above, there is a need to makeoptical systems composed of several micro-optical components and inwhich the tolerances needed are much tighter than can be achieved withconventional approaches. In addition to requiring tight tolerances,elements of lower cost are also demanded. The alignment tolerance neededmay be 1 micron to 5 microns, which is very expensive to achieve withconventional methods.

To achieve greater alignment tolerances, passive alignment techniqueshave been used as set forth in U.S. Pat. No. 5,683,469 to Feldmanentitled “Microelectronic Module Having Optical and ElectricalInterconnects”. One such passive alignment technique is to place metalpads on the optics and on the laser and place solder between them anduse self-alignment properties to achieve the alignment. When solderreflows, surface tension therein causes the parts to self-align.However, passive alignment has not been employed for wafer-to-waferalignment. In particular, the high density of solder bumps required andthe thickness and mass of the wafer make such alignment impractical.

Another problem in integrating multiple optical elements formed onseparate wafers at a wafer level arises due to the dicing process forforming the individual integrated elements. The dicing process is messydue to the use of a dicing slurry. When single wafers are diced, thesurfaces thereof may be cleaned to remove the dicing slurry. However,when the wafers are bonded together, the slurry enters the gap betweenthe wafers. Removing the slurry from the gap formed between the wafersis quite difficult.

Integrated elements are also sometimes made by injection molding. Withinjection molding, plastic elements can be made having two moldedelements located on opposite sides of a substrate. Multiple plasticelements can be made simultaneously with a multi-cavity injectionmolding tool.

Glass elements are also sometimes made by molding, as in U.S. Pat. No.4,883,528 to Carpenter entitled “Apparatus for Molding Glass OpticalElements”. In this case, just as with plastic injection molding,multiple integrated elements are formed by molding two elements onopposite sides of a substrate. Glass molding however has disadvantagesof being expensive to make tooling and limited in size that can be used.

To make optics inexpensive, replication techniques are typically used.In addition to plastic injection molding and glass molding discussedabove, individual elements may also be embossed. An example of suchembossing may be found in U.S. Pat. No. 5,597,613 to Galarneau entitled“Scale-up Process for Replicating Large Area Diffractive OpticalElements”. Replicated optics have not been used previously together withsolder self-alignment techniques. For each replication method, manyindividual elements are generated as inexpensively as possible.

Such replication processes have not been used on a wafer level withsubsequent dicing. This is primarily due to the stresses imposed on theembossed layer during dicing. When using embossing on a wafer level,unique problems, such as keeping the polymer which has been embossedsufficiently attached to the substrate, e.g., such that the alignment,especially critical on the small scale or when integrating more than oneelement, is not upset.

Further, these replication processes are not compatible with the waferlevel photolithographic processes. In particular, replication processesdo not attain the required alignment accuracies for photolithographicprocessing. Even if embossing was compatible with lithographicprocessing, it would be too expensive to pattern lithographically on oneelement at a time. Further, the chemical processing portion oflithographic processing would attack the embossing material.

Other problems in embossing onto plastic, as is conventionally done, andlithographic processing arise. In particular, the plastic is alsoattacked by the chemicals used in lithographic processing. Plastic alsois too susceptible to warping due to thermal effects, which isdetrimental to the alignment required during lithographic processing.

SUMMARY OF THE INVENTION

Considering the foregoing background, it is an object of the presentinvention to efficiently produce integrated multiple optical elements.Such efficient production is accomplished by forming the integratedmultiple optical elements on a wafer level.

It is further an object of the present invention to address the problemsarising when attempting to achieve such wafer level production ofintegrated multiple optical elements. These problems include ensuringaccurate alignment, allowing precise dicing of the wafer to theconstituent integrated multiple optical elements when more than onewafer is bonded together, and providing additional features for allowingeasy incorporation of the integrated multiple optical element into anoverall system for a desired application.

It is another object of the present invention to provide embossing whichhas sufficient alignment for use with photolithographic features andsufficient adhesion to withstand dicing.

These and other objects of the present invention will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description givesspecific examples, while indicating the preferred embodiments of thepresent invention, are given by way of illustration only, since variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

FIG. 1 illustrates a first embodiment for bonding together two wafers;

FIG. 2 illustrates a second embodiment for bonding together two wafers;

FIG. 3 a is a perspective view illustrating wafers to be bonded;

FIG. 3 b is a top view illustrating an individual die on a wafer to bebonded;

FIGS. 4A-4B illustrate specific examples of bonding two substratetogether;

FIG. 5 is a flow chart of the bonding process of the present invention;

FIG. 6A illustrates embossing an embossable material onto a supportsubstrate using a mater element;

FIG. 6B illustrates embossing an embossable material on a supportsubstrate using a master element;

FIG. 7 illustrates a wafer on which optical elements have been formed onboth sides; and

FIG. 8 is a cross-sectional view of a substrate having a hybrid elementconsisting of a microlens with a diffractive element integrated directlythereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As can be seen in FIG. 1, a first substrate wafer 10 and a secondsubstrate wafer 12 are to be bonded together in order to provideintegrated multiple optical elements. A wafer is a disc, typically 4, 6,8, or 12 inches in diameter and typically having a thickness between 400microns and 6 mm.

These wafers have an array of respective optical elements formed thereonon either one or both surfaces thereof. The individual optical elementsmay be either diffractive, refractive or a hybrid thereof. Dashed lines8 indicate where the dicing is to occur on the wafers to provide theindividual integrated elements.

A bonding material 14 is placed at strategic locations on eithersubstrate in order to facilitate the attachment thereof. By surroundingthe optical elements which are to form the final integrated die, theadhesive 14 forms a seal between the wafers at these critical junctions.During dicing, the seal prevents dicing slurry from entering between theelements, which would result in contamination thereof. Since theelements remain bonded together, it is nearly impossible to remove anydicing slurry trapped therebetween. The dicing slurry presents even moreproblems when diffractive elements are being bonded, since thestructures of diffractive elements tend to trap the slurry.

Preferably, an adhesive or solder can be used as the bonding material14. Solder is preferable in many applications because it is smootherthan adhesives and allows easier movement prior to bonding. Adhesiveshave the advantages of being less expensive for a number ofapplications, they can be bonded with or without heating, they do notsuffer with oxidation, and they can be transparent.

When using a fluid adhesive as the bonding material, the viscosity ofthe fluid adhesive is important. The adhesive cannot be too thin, orelse it beads, providing indeterminant adhesion, allowing the dicingslurry to get in between the elements on the wafers, therebycontaminating the elements. The adhesive cannot be too thick, or therestoring force is too great and sufficient intimate contact between thesubstrates 10 and 12 to be bonded is not achieved. The fluid adhesivepreferably has a viscosity between 1,000 and 10,000 centipoise.Satisfactory epoxies include Norland 68 and Masterbond UV 15-7.

When a fluid adhesive is employed, it must be provided in a controlledmanner, such as ejected from a nozzle controlled in accordance with thedesired coordinates to receive the fluid adhesive. After alignment ofthe wafers, the entire assembly is cured, thereby hardening the fluidadhesive and completing the bonding.

When solder is used, an electroplating or sputtering process may beemployed. For example, a masking material may be put over the substratewherever the substrate is not to have solder. Then the entire wafer isplaced into a bath or sputtering chamber. Then solder is placed over theentire wafer and the masking material is pulled off, leaving solderwhere there was no masking material. Once the wafers are appropriatelyaligned, the solder is then heated up to reflow. The solder is cooledand allowed to re-harden, thereby completing the bond.

When using the bonding material used alone as shown in FIG. 1 is a fluidadhesive, a more viscous adhesive is needed in order to ensure that thebonding material remains where it is deposited. Even using a viscousadhesive, the adhesive still typically spreads over a relatively largearea, resulting in a need for a larger dead space between elements to beintegrated to accommodate this spread without having the adhesiveinterfere with the elements themselves.

It is also difficult to control the height of the adhesive when theadhesive is used alone. This results in the amount of adhesive beingovercompensated and the height of the adhesive, and hence the separationbetween the wafers, often being greater than desired. The difficultycontrolling the height of the adhesive also results in air being trappedwithin the space containing the optical elements. This arises from theuncertainty as to the height and the timing of when a vacuum is pulledon the wafer pair. This air is undesirable, as it may expand uponheating and disrupt the bond of the elements.

Therefore, an advantageous alternative is shown in FIG. 2, in which onlyan individual integrated optical element of the wafer is shown. Standoffs 16 for each element to be integrated are etched or replicated intothe bottom substrate wafer 12 at the same time the array of opticalelements are made for the substrate wafer 12, and typically will be ofthe same material as the substrate wafer. These stand offs 16 preferablyinclude a trench formed between two surfaces in which the adhesive 14 isto be placed. These trenches then provide precise spacing between thesubstrates to be bonded and provide more of a bonding surface to whichthe adhesive 14 can adhere. This increased surface area also reducesbeading problems.

When solder is used as the bonding material 14, solid stand-offs arepreferably used to provide the desired separation between the wafers.The solder is then deposited in a thin, e.g., 4-5 micron, layer on topof the stand-offs. While the solder could be used alone as shown in FIG.1, it is more feasible and economical to use the solder in conjunctionwith stand-offs.

The use of the stand-offs allows a more uniform and predictable heightto be obtained, resulting in less air being trapped between the bondedelements. A vacuum may now be pulled just before or at contact betweenthe bonding material and the other substrate, due to the reduction invariability of the separation.

The substrate not containing the stand-offs may have notches formedthereon to receive the stand-offs 16 therein. These notches can beformed at the same time any optical elements on that surface are formed.In such a configuration, the stand-offs 16 and the corresponding notcheswill serve as alignment features, facilitating alignment of the wafersto one another.

FIG. 3 a shows the two substrates 10 and 12 prior to being bonded anddiced. The individual optical elements 19 to be integrated may consistof one or more optical elements. Further, the optical elements on thewafers may be identical, or may differ from one another. Prior tojoining the wafers 10, 12, the bonding material 14 is placed on at leastone of the wafers in the manner described above. Advantageously, bothsubstrates 10 and 12 include fiducial marks 18 somewhere thereon, mostlikely at an outer edge thereof, to ensure alignment of the wafers sothat all the individual elements thereon are aligned simultaneously.Alternatively, the fiducial marks 18 may be used to create mechanicalalignment features 18′ on the wafers 11, 12. One or both of the fiducialmarks 18 and the alignment features 18′ may be used to align the wafers.

FIG. 3 b shows a top view of a substrate 12 to be bonded including thelocation of the surrounding bonding material 14 for a particular element19. As can be seen from this top view, the bonding material 14 is tocompletely surround the individual optical element, indicated at 19.

For either embodiment shown in FIG. 1 or 2, the bonding materialprovided either directly or using stand-offs completely seals eachelement to be individually utilized. Thus, when dicing a wafer in orderto perform the individual elements, dicing slurry used in the dicingprocess is prevented from contaminating the optical elements. Thus, inaddition to providing a structural component to maintain alignment andrigidity during dicing, the bonding material seal also makes the dicinga much cleaner process for the resultant integrated dies.

A specific example of integrated multiple optical elements is shown inFIG. 4A. A refractive 20 is formed on a surface of the first substrate12. A diffractive 22 is formed on a surface of the other substrate 10. Adiffractive 28 may also be formed on a bottom surface of eithersubstrate. The stand offs 16 forming the trenches for receiving theadhesive 14 are formed at the same time as a refractive lens.

Another specific example of integrated multiple optical elements isshown in FIG. 4B. An active element 25, e.g., a laser, is provided onthe first substrate 12. The first substrate 12 may be etched to providea reflective surface 27, 17 therein. The second substrate 11, which hasbeen separated to form dies with diffractive elements 22 thereon, may bemounted to the first substrate 12 via the adhesive 12. Stand-offs 29 maybe provided to insure alignment between the reflective surface 27, 17,the active element 25 and the diffractive element 22.

When the lens 20 on the wafer 12 is directly opposite the other wafer,the vertex of the lens 20 may also be used to provide the appropriatespacing between the substrates 10 and 12. If further spacing isrequired, the stand offs 16 may be made higher to achieve thisappropriate spacing.

In addition to using the fiducial marks 18 shown in FIG. 3 a foralignment of the substrates 10, 12, the fiducial marks 18 may also beused to provide metalized pads 24 on opposite sides of the substratesrather than their bonding surfaces in order to facilitate alignment andinsertion of the integrated multiple optical element into its intendedend use. Such metal pads are particularly useful for mating theintegrated multiple optical elements with an active or electricalelement, such as in a laser for use in an optical head, a laser pointer,a detector, etc. Further, for blocking light, metal 26 may be placed onthe same surface as the diffractive 22 itself using the fiducial marks18.

FIG. 5 shows a flow chart of the general process of bonding together twowafers in accordance with the present invention. In step 30, a substratewafer is positioned relative to the bonding material to be distributed.In step 32, the bonding material is applied to the wafer in a pattern toprovide sealing around the optical elements, either directly or with thestand-offs 16. In step 34, the second substrate wafer is aligned withthe first substrate wafer. Just before contact is achieved, a vacuum ispulled to remove air from between the substrates. In step 36, the wafersare brought into contact. In step 38, the alignment of the two wafers isconfirmed. In step 40, the adhesive is cured or the solder is reflowedand then allowed to harden. Once firmly bonded, in step 42, the bondedwafers are diced into the individual elements.

The elements to be bonded together are preferably created by directphotolithographic techniques, as set forth, for example, in U.S. Pat.No. 5,161,059 to Swanson, which is hereby incorporated by reference, forthe diffractive optical elements, or in creating the sphericalrefractive elements by melting a photoresist as taught in O. Wada,“Ion-Beam Etching of InP and its Application to the Fabrication of HighRadiance InGAsP/InP Light Emitting Diodes,” General Electric ChemicalSociety, Solid State Science and Technology, Vol. 131, No. 10, October1984, pages 2373-2380, or making refractive elements of any shapeemploying photolithographic techniques used for making diffractiveoptical elements when the masks used therein are gray scale masks suchas high energy beam sensitive (HEBS) or absorptive gray scale masks,disclosed in provisional application Ser. No. 60/041,042, filed on Apr.11, 1997, which is hereby incorporated by reference.

Alternatively, these photolithographic techniques may be used to make amaster element 48 in glass which in turn may then be used to stamp outthe desired element on a wafer level in a layer of embossable material50 onto a substrate 52 as shown in FIG. 6B. The layer 50 is preferably apolymer, while the substrate 52 is can be glass, e.g., fused silica, orplastic, preferably polycarbonate or acrylic. The polymer is preferablya UV curable acrylate photopolymer having good release from a master andgood adherence to a substrate such that it does not crack after cure orrelease from the substrate during dicing. Suitable polymers includePHILIPS type 40029 Resin or GAFGARD 233. Dashed lines 58 indicate thedicing lines for forming an individual integrated element from thewafer.

In the embodiment shown in FIG. 6A, the layer of embossable material 50is provided on the master element 48. A layer of adhesion promoter 54 ispreferably provided on the substrate 52 and/or a layer of a releaseagent is provided on the master element 48 in between the master elementand the embossing material. The use of an adhesion promoter and/orrelease agent is of particular importance when the master and thesubstrate are of the same material or when the master naturally has ahigher affinity for adhesion to the embossable material.

The type of adhesion promoter used is a function of the photopolymer tobe used as the embossable material, the master material and thesubstrate material. A suitable adhesion promoter for use with a glasssubstrate is HMDS (hexamethyl disilizane). This adhesion promoterencourages better bonding of the embossable material onto the substrate52, which is especially critical when embossing on the wafer level,since the embossed wafer is to undergo dicing as discussed below.

The provision of the embossable layer 50 on the master 48 and of theadhesion promoting layer 54 on the substrate 52 advantageously providessmooth surfaces which are to be brought into contact for the embossing,making the elimination of air bubbles easier as noted below. Theprovision of the embossable layer on the master 48 also provides aconvenient mechanism for maintaining alignment of contacted, alignedwafer which have not been bonded, as discussed below.

If either the substrate or the master is made of plastic, it ispreferable to place the polymer on the other non-plastic component,since plastic absorbs strongly in the UV region used for activating thepolymer. Thus, if the UV radiation is required to pass through plastic,a higher intensity beam will be required for the desired effect, whichis clearly less efficient.

The use of embossing on the wafer level is of particular interest whenfurther features are to be provided on the wafer using lithographicprocesses, i.e., material is selecting added to or removed from thewafer. Such further features may include anti-reflective coatings orother features, e.g. metalization pads for aligning the die diced fromthe substrate 52 in a system, on the embossed layer. Any such featuresmay also be lithographically provided on an opposite surface 56 of thesubstrate 52.

Typically an anti-reflective coating would be applied over the entiresurface, rather than selectively. However, when using both ananti-reflective coating and metal pads, the metal would not adhere aswell where the coating is present and having the coating covering themetal is unsatisfactory. Further, if the wafer is to be bonded toanother wafer, the bonding material would not adhere to the surface ofhaving such an anti-reflective coating, thereby requiring the selectivepositioning of the coating.

For achieving the alignment needed for performing lithographicprocessing in conjunction with the embossing, fiducial marks as shown inFIG. 3 may be provided on both the substrate 52 and the master 48. Whenperforming lithographic processing, the alignment tolerances requiredthereby make glass more attractive for the substrate than plastic. Glasshas a lower coefficient of thermal expansion and glass is flatter thanplastic, i.e., it bows and warps less than plastic. These features areespecially critical when forming elements on a wafer level.

When placing the master on the substrate, the wafer cannot be broughtstraight down into contact. This is because air bubbles which adverselyaffect the embossed product would be present, with no way of removingthem.

Therefore, in bringing the master into contact with the substrate, themaster initially contacts just on one edge of the substrate and then isrotated to bring the wafer down into contact with the substrate. Thisinclined contact allows the air bubbles present in the embossablematerial to be pushed out of the side. Since the master is transparent,the air bubbles can be visually observed, as can the successfulelimination thereof. As noted above, it is the presence of these airbubbles which make it advantageous for the surfaces to be brought intocontact be smooth, since the diffractive formed on the surface of themaster 48 could trap air bubbles even during such inclined contact.

The degree of the inclination needed for removing the air bubblesdepends on the size and depth of the features being replicated. Theinclination should be large enough so that the largest features are nottouching the other wafer across the entire wafer on initial contact.

Alternatively, if the replica wafer is flexible, the replica wafer maybe bowed to form a slightly convex surface. The master is then broughtdown in contact with the replica wafer in the center and then thereplica wafer is released to complete contact over the entire surface,thereby eliminating the air bubbles. Again, the amount of bow requiredis just enough such that the largest features are not touching the otherwafer across the entire wafer on initial contact.

When using the fiducial marks themselves to align the master element 48to the glass substrate 52 in accordance with the present invention, aconventional mask aligner may be used in a modified fashion. Typicallyin a mask aligner, a mask is brought into contact with a plate and thena vacuum seals the mask and plate into alignment. However, a vacuumcannot be created when a liquid, such as a polymer, embossable materialis on top of a wafer. Therefore, the above inclined contact is used.Once contact is established, the wafers are aligned to one another in aconventional fashion using the fiducial marks before being cured.

Further, the intensity required to cure the polymer is very high, e.g.,3-5 W/cm², and needs to be applied all at once for a short duration,e.g., less than 30 seconds. If enough energy and intensity are notapplied at this time, hardening of the polymer can never be achieved.This is due to the fact that the photoinitiators in the polymer may beconsumed by such incomplete exposure without full polymerization.

However, it is not easy to provide such a high intensity source with themask aligner. This is due both to the size and the temperature of thehigh energy light source required. The heat from the high energy sourcewill cause the mask aligner frame to warp as it is exposed to thermalvariations. While the mask aligner could be thermally compensated orcould be adapted to operate at high temperatures, the following solutionis more economical and provides satisfactory results.

In addition to the inclined contact needed for placing the master infull contact with the substrate in the mask aligner, once such fullcontact is achieved, rather than curing the entire surface, a deliverysystem, such as an optical fiber, supplies the radiation from a UVsource to the master-substrate in contact in the mask aligner. Thedelivery system only supplies UV radiation to individual spots on thepolymer.

The delivery system is small enough to fit in the mask aligner and doesnot dissipate sufficient heat to require redesign of the mask aligner.When using an optical fiber, these spots are approximately 2 mm.Alternatively, a UV laser which is small and well contained, i.e., doesnot impose significant thermal effects on the system, may be used.

The delivery system provides the radiation preferably to spots in theperiphery of the wafer in a symmetric fashion. For a 4 inch wafer, onlyabout 6-12 spots are needed. If additional spots are desired forincreased stability, a few spots could be placed towards the center ofthe wafer. These spots are preferably placed in the periphery and aminimal number of these spots is preferably used since an area where atack spot is located does not achieve as uniform polymerization as theareas which have not been subjected to the spot radiation.

These tack spots tack the master in place with the substrate. Theillumination used for curing the tack spots is only applied locally andthere are few enough of these tack spots such that the area receivingthe illumination is small enough to significantly affect the rest of theembossable material. Once alignment has been achieved and the mastertacked into place, the substrate-master pair is removed from the alignerand then cured under the high intensity UV source over the entiresurface for full polymerization. The tack spots prevent shifting of thealignment achieved in the mask aligner, while allowing thesubstrate-master pair to be removed from the mask aligner to thereby usethe high energy light source external to the mask aligner for curing thepolymer.

Alternatively, the fiducial marks may be used to form mechanicalalignment features on the perimeter of the surfaces to be contacted. Themechanical alignment features may provide alignment along any axis, andthere may be more than one such mechanical alignment feature. Forexample, the stand-offs in FIG. 4 are for aligning the wafers along they axis, while the metal pads provide alignment of the wafer pair toadditional elements along the x and z axes. The alignment features arepreferably formed by the embossing itself.

The embossing and the lithographic processing on the opposite surfacemay be performed in either order. If the embossing is performed first,it is advantageous to leave the master covering the embossed layer untilthe subsequent processing on the opposite surface is complete. Themaster will then act as a seal for the embossed structure, protectingthe polymer from solvents used during lithographic processing andkeeping the features accurate throughout heating during lithographicprocessing.

If the lithographic processing is performed first, then more precisealignment is required during embossing to provide sufficient alignmentto the photolithographic features than is required during normalembossing. Thus, embossing equipment is not set up to perform suchalignment. Then, the above alignment techniques are required duringembossing.

Once all desired processing has been completed, the wafer is diced toform the individual elements. Such dicing places mechanical stresses onthe embossed wafer. Therefore, full polymerization and sufficientadhesion of the embossed portion to the substrate is of particularimportance so that the embossed portion does not delaminate duringdicing. Therefore, care must be taken in selecting the particularpolymer, an adhesion promoter, and the substrate, and how these elementsinteract. Preferably, in order to avoid delamination of the embossedlayer during dicing, the adhesion of the polymer to the substrate shouldbe approximately 100 grams of shear strength on a finished die.

When both wafers to be bonded together as shown in FIGS. 1-4 have beenembossed with a UV cured polymer, the typical preferred use of a UVepoxy for such bonding may no longer be the preferred option. This isbecause the UV cured polymer will still highly absorb in the UV region,rendering the available UV light to cure the epoxy extremely low, i.e.,in order to provide sufficient UV light to the epoxy to be cured, theintensity of the UV light needed is very high. Therefore, the use ofthermally cured resin to bond such wafers is sometimes preferred.

Alternatively, polymer on the portions not constituting the elementsthemselves may be removed, and then the UV epoxy could be employed inthese cleared areas which no longer contain the UV polymer to directlybond the glass substrate wafer having the UV polymer with another wafer.A preferably way to remove the polymer includes provides a pattern ofmetal on the master. This metal blocks light, thereby preventing curingof the polymer in the pattern. When a liquid polymer is used, thisuncured polymer may then be washed away. Other materials, such as the UVepoxy for wafer-to-wafer bonding or metal for active element attachmentor light blocking, may now be placed where the polymer has been removed.

In addition to the bonding of the two substrates shown in FIGS. 1-4, thealignment marks may be used to produce optical elements on the otherside of the substrate itself, at shown in FIG. 7. The creation may alsooccur by any of the methods noted above for creating optical elements.The double sided element 70 in FIG. 7 has a diffractive element 72 on afirst surface 70 a thereof and a refractive element 74 on a secondsurface 70 b thereof, but any desired element may be provided thereon.Again, metal pads 76 may be provided through lithographic processing onthe hybrid element.

A further configuration of an integrated multiple optical elements isshown in FIG. 8, in which a diffractive element 82 is formed directly ona refractive element 84. The refractive element may be made by any ofthe above noted photolithographic techniques. In the specific exampleshown in FIG. 8, the refractive element is formed by placing a circularlayer of photoresist 86 on a surface of optical material using a mask.The photoresist is then partially flowed using controlled heat so thatthe photoresist assumes a partially spherical shape 87. Thereafter, thesurface is etched and a refractive element 84 having substantially thesame shape as the photoresist 87 is formed by the variable etch rate ofthe continually varying thickness of the photoresist 87. The microlens84 is then further processed to form the diffractive element 82 thereon.The diffractive element may be formed by lithographic processing orembossing using an embossed layer 85.

The wafers being aligned and bonded or embossed may contain arrays ofthe same elements or may contain different elements. Further, whenalignment requirements permit, the wafers may be plastic rather thanglass. The integrated elements which are preferred to be manufactured onthe wafer level in accordance with the present invention are on theorder of 100 microns to as large as several millimeters, and requirealignment accuracies to ±1-2 microns, which can be achieved using thefiducial marks and/or alignment features of the present invention.

When the optical elements are provided on opposite surfaces of asubstrate, rather than bonded facing one another, tolerable alignmentaccuracies are ±10 microns. This is due to the fact that when light istransmitted through the thickness of the glass, slight amounts of tiltcan be corrected or incorporated.

As an alternative to the fiducial marks used for passive alignment, thefiducial marks may be used to create mechanical alignment features, suchas corresponding groves joined by a sphere, metalization pads joined bya solder ball, and a bench with a corresponding recess. Only a few ofthese alignment features is needed to align an entire wafer.

All of the elements of the present invention are advantageously providedwith metalized pads for ease of incorporation, including alignment, intoa system, typically including active elements. The metalized pads mayefficiently be provided lithographically on the wafer level.

The invention being thus described, it would be obvious that the samemay be varied in many ways. Such variations are not regarded as adeparture from the spirit and scope of the invention, and suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method of making optical elements on a wafer level comprising:making a master including a plurality of optical elements on asubstantially planar surface thereof; simultaneously imprinting areplica of said plurality of optical elements in an imprintable materialby applying the substantially planar surface of the master to theimprintable material; providing a support substrate for the replica, thesupport substrate having first and second opposing parallel surfaces,the replica being on one of the first and second opposing parallelsurfaces; hardening the imprintable material to form a hardened replica,the hardening including hardening only portions of the imprintablematerial corresponding to the optical elements, such that regions nothaving the plurality of optical elements thereon include unhardenedimprintable material; removing the master from the hardened replica;removing imprintable material from regions not having the plurality ofoptical elements thereon to form cleared areas; and separating thesupport substrate through the first and second opposing parallelsurfaces in the cleared areas to form individual optical elements, eachindividual optical element including the hardened replica and thesupport substrate.
 2. The method as recited in claim 1, furthercomprising providing imprintable material in a thin film on thesubstantially planar surface of the master prior to the imprinting. 3.The method as recited in claim 1, wherein imprinting includes: providingthe imprintable material to the substantially planar surface of themaster; providing an adhesion promoter on the support substrate; andcontacting the substantially planar surface of the master having theimprintable material thereon to a surface of the substrate having theadhesion promoter thereon.
 4. The method as recited in claim 1, whereinthe support substrate includes fiducial marks, the method furthercomprising aligning the master to the fiducial marks.
 5. The method asrecited in claim 1, further comprising coating the replica with ananti-reflective coating.
 6. The method according to claim 1, furthercomprising selectively removing or adding to the optical element in apredetermined pattern.
 7. The method according to claim 1, furthercomprising selectively removing or adding to the support substratebefore imprinting.
 8. The method according to claim 6, whereinselectively removing or adding occurs after imprinting.
 9. The methodaccording to claim 6, wherein selectively removing or adding includesproviding metal pads on another of the first and second opposingparallel surfaces opposite where the replica is to be located.
 10. Themethod as recited in claim 1, wherein imprinting includes imprintingboth sides of the support substrate, such that a plurality of pairs ofopposing, corresponding optical elements are formed on the supportsubstrate, each individual optical element including a pair of opposing,corresponding optical elements.
 11. The method as recited in claim 10,wherein a different master is used for imprinting either side of saidboth sides.
 12. The method as recited in claim 11, wherein a firstmaster includes diffractive optical elements and a second masterincludes refractive optical elements.
 13. The method as recited in claim1, wherein said master is a wafer.
 14. The method as recited in claim 6,wherein said selectively removing or adding is lithographic.
 15. Themethod as recited in claim 6, wherein said selectively removing oradding includes selectively removing imprintable material.
 16. Themethod as recited in claim 1, wherein removing the imprintable materialincludes providing metal in a pattern on said master and, after saidimprinting, removing away unhardened imprintable material overlappingthe metal.
 17. The method as recited in claim 13, further comprisingproviding fiducial marks on both the wafer master and the supportsubstrate.
 18. The method according to claim 13, further comprising:confirming alignment of the support substrate and the wafer master in amask aligner; and tacking together the support substrate and wafermaster at discrete locations once alignment is confirmed.
 19. The methodaccording to claim 18, further comprising removing the support substrateand the wafer master from the mask aligner after said tacking and thenhardening the imprintable material.
 20. The method according to claim 1,wherein said imprinting includes initially bringing the master intocontact with only a portion of the support substrate, with theimprintable material on at least one of the master and the supportsubstrate, and then bringing the entire master into contact with thesupport substrate with the imprintable material therebetween.
 21. Themethod as recited in claim 18, wherein said tacking includes providinglocalized fixing of said imprintable material.
 22. The method accordingto claim 1, wherein said imprinting includes initially bringing aportion of the substantially planar surface of the master into contactwith a center of the support substrate, with the imprintable material onat least one of the master and the support substrate, and then bringingthe entire substantially planar surface of the master into contact withthe support substrate with the imprintable material therebetween. 23.The method according to claim 1, wherein said imprinting includesinitially bringing a portion of the substantially planar surface of themaster into contact with an edge of the support substrate, with theimprintable material on at least one of the master and the supportsubstrate, and then bringing the entire substantially planar surface ofthe master into contact with the support substrate with the imprintablematerial therebetween.
 24. The method according to claim 1, wherein thehardened replica is adhered to the support substrate sufficiently toavoid delamination therefrom during said separating.
 25. The methodaccording to claim 2, wherein said imprintable material covers theplurality of optical elements, providing a substantially smooth uppersurface on the substantially planar surface of the master.
 26. Themethod as recited in claim 1, further comprising lithographicallycreating a plurality of optical lithographs on another of the first andsecond opposing parallel surfaces opposite where the replica is to belocated, one of the optical lithograph and the replica being adiffractive optical element and another being a refractive opticalelement.
 27. The method as recited in claim 26, wherein each individualoptical element includes a corresponding optical lithograph.
 28. Themethod as recited in claim 1, further comprising: lithographicallycreating a plurality of optical lithographs on another substrate; andsecuring the substrate having the replicas and the another substratehaving the optical lithographs thereon before separating, one of theoptical lithograph and the replica being a diffractive optical elementand another being a refractive optical element.
 29. The method asrecited in claim 28, wherein each individual optical element includes acorresponding optical lithograph.
 30. The method as recited in claim 28,wherein securing occurs at substantially planar regions of thesubstrates.
 31. The method as recited in claim 28, wherein securingincludes providing a separation mechanism between the substrates. 32.The method according to claim 1, wherein removing the imprintablematerial includes removing only the unhardened imprintable material. 33.The method according to claim 32, wherein removing only the unhardenedmaterial is performed before separating the support substrate intoindividual optical elements.
 34. The method according to claim 1,wherein the hardening includes providing metal on the master only inregions not having the plurality of optical elements thereon, such thatthe regions not having the plurality of optical elements thereon andoverlapping the metal include unhardened imprintable material.